House Ways and Means Committee Chairman Rep. Dave Camp , R-Mich., center, flanked by the committee's ranking member Rep. Sander Levin, D-Mich., right, and Rep. Sam Johnson, R-Texas, questions Internal Revenue Service (IRS) Commissioner John Koskinen as he appears before the committee on Capitol Hill in Washington, Friday, June 20, 2014, in their continuing probe of whether tea party groups were improperly targeted for increased scrutiny by the IRS. The IRS asserts it can't produce emails from seven officials connected to the tea party investigation because of computer crashes, including the emails from Lois Lerner, the former IRS official at the center of the investigation who has invoked her Fifth Amendment right at least nine times to avoid answering lawmakers' questions. (AP Photo/J. Scott Applewhite)

This March 5, 2014 file photo shows former Internal Revenue Service (IRS) official Lois Lerner speaks on Capitol Hill in Washington. The Internal Revenue Service has lost more emails connected to the tea party investigation, congressional investigators said Tuesday. The IRS said last Friday it had lost an untold number of emails when Lois Lerner's computer crashed in 2011. Lerner used to head the division that handles applications for tax-exempt status. (AP Photo/Lauren Victoria Burke, File)

WASHINGTON (AP) — Eight federal employees connected to the tea party investigation experienced hard drive crashes, resulting in an unknown number of lost emails, Internal Revenue Service Commissioner John Koskinen told lawmakers Friday in an unusually tense congressional hearing.

A week ago the IRS acknowledged it could not produce some of the emails of the IRS executive at the center of the probe because her computer crashed in 2011. Koskinen acknowledged to lawmakers that the hard drive was recycled and presumably destroyed.

"I want that hard drive and I want the hard drive of every computer that crashed," said the chairman of the House Ways and Means Committee, Rep. Dave Camp, R-Mich.

Koskinen said the IRS took extra measures to try to retrieve the lost emails. But he was unapologetic about the computer crashes or the period when the IRS advised Congress that emails it had sought were lost.

"I don't think an apology is owed," Koskinen said.

Koskinen says it's not clear whether all eight of the hard drive crashes resulted in lost emails.

Koskinen also said appointment of a special federal prosecutor to investigate the IRS handling of tax-exempt applications would be a "monumental waste of taxpayer funds."

The congressional investigation has been highly politicized because of allegations that the IRS improperly singled out tea party groups seeking tax-exempt status. Friday's hearing was unusually tense, as Camp and other Republicans occasionally interrupted Koskinen and continued to ask other questions before Koskinen had an opportunity to answer.

The senior Democrat on the committee, Rep. Sander Levin of Mich., chided his colleagues that, "Witnesses deserve some respect."

An FBI investigation is ongoing.

The former IRS official at the center of the investigation, Lois Lerner, has invoked her Fifth Amendment right at least nine times to avoid answering lawmakers' questions. Lerner did not learn that IRS staffers were improperly reviewing applications of tea party and other conservative groups for tax-exempt status until weeks after her computer crashed, according to an earlier audit by the Treasury Department inspector general for tax administration.

Lerner's computer crashed sometime around June 13, 2011, according to emails provided to Congress. She first learned about the tea party reviews on June 29, according to the inspector general.

Koskinen told Congress that Lerner's hard drive was unavailable to them because it had been recycled.

The IRS said last week it became aware of the missing emails in February of this year. The IRS did not know whether the other computer crashes have resulted in lost emails as well. It will also not say how often its computers fail and lose data.

The lost emails are raising questions even by the government's records officer. In a June 17 letter to the IRS, Paul Wester Jr. asked the agency to investigate the loss of records and whether any disposal of data was authorized. Wester, the chief records officer at the National Archives and Records Administration, was responding to the IRS' June 13 disclosure of Lerner's lost emails.

Wester's letter did not address the lost records of six other employees that the IRS disclosed that day. Wester said the IRS is required to report its finding within 30 days. Federal agencies are supposed to report destruction of records — whether accidental or intentional — to the National Archives "promptly" after an incident.

The IRS said that after Lerner's computer crashed in June 2011, technicians were not able to retrieve data from her hard drive.

In May, more than two months after the IRS discovered the emails were missing, the IRS assured Camp that it would provide all applications from groups seeking tax-exempt status in 2010 and 2011, including all files, correspondence and internal IRS records related to them. Camp had asked for the records in May 2012.

It's similarly unclear why the IRS didn't attempt to recover the emails from backup servers in June 2011, especially since Lerner told an IRS computer technician in a July 2011 email, "There were some documents in the files that are irreplaceable."

Shawn Henry, the FBI's former cyber director, said technicians should have been able to retrieve data from the servers around the times the computers crashed.

"If they knew there was a problem in 2011," said Henry, now president of CrowdStrike, a security technology company, "they could have or should have been able to recover it."

The IRS told Congress last week that recovering emails has been a challenge because doing so is "a more complex process for the IRS than it is for many private or public organizations."

The IRS was able to find copies of 24,000 Lerner emails from between 2009 and 2011 because Lerner had sent copies to other IRS employees. Overall, the IRS said it was producing 67,000 emails to and from Lerner, covering 2009 to 2013. The agency said it searched for emails of 83 people and spent nearly $10 million to produce hundreds of thousands of documents.

At the time that Lerner's computer crashed, IRS policy had been to make copies of all IRS employees' email inboxes every day and hold them for six months. The agency changed the policy in May 2013 to keep these snapshots for a longer, unspecified amount of time. Had this been the policy in 2011, when at least two of the computer crashes occurred, there likely could have been backups of the lost emails today.

The chief executive for an email-archiving company, Pierre Villeneuve of Jatheon Technologies, said most public and private sector organizations keep emails for several years, not six months, because of financial regulations and inexpensive computer storage.

"To have a large agency like the IRS have a very weak policy for email archiving and retention is quite shocking," Villeneuve said. "If this were a private enterprise and they couldn't produce this information on demand, they'd be in trouble. They'd either be fined or accused of hiding information."

The IRS has said technicians sent Lerner's hard drive to a forensic lab run by the agency's criminal investigations unit. But the information was not recoverable, a technician told her in an Aug. 5, 2011, email.

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